1. Field of the Invention
The field of our invention relates generally to semiconductor manufacturing, and more particularly to a method for cladding an interconnect for improved electromigration performance.
2. Description of Related Art
During the process of semiconductor fabrication, metal films are deposited on a wafer surface. After being patterned into various metal strips or lines utilizing, for example, a photomasking process, a metal layer then connects the devices on the wafer as required by the circuit operation. These strips of metal are often referred to as interconnects.
Interconnects are impacted as the semiconductor industry moves toward smaller and smaller device dimensions in order to allow for a greater density of devices per wafer. As the device dimensions shrink, the width of the interconnects also shrinks. Therefore, the narrow dimensions of an electrical interconnect lead to higher resistances and greater current densities in the conductive layer which forms the electrical interconnect. The increased current density then causes a reduction in the electromigration lifetime.
Electromigration is a phenomenon that occurs in aluminum interconnects while the circuit is in operation. Electromigration shows up as a field failure, and not as a failure during semiconductor processing. Electromigration is caused by the diffusion of the aluminum atoms in electrical fields set up in the interconnect while the circuit is in operation. Electromigration is often the result of the movement of the electrons in a conductor when an electrical field is raised to a very high level and the operating temperature is high. This results in a physical displacement of conductor atoms, such as aluminum atoms, that eventually leads to an open conductor and circuit failure when the metal thins and eventually separates completely. Electromigration becomes even more of a problem as the level of integration increases. This is because the higher number of components in a VLSI (Very Large Scale Integration) circuit creates more current flow and also generates more heat.
One solution to increasing the electromigration lifetime of an aluminum interconnect is to introduce an alloying species, for example, typically 0.5% copper to the aluminum. Another solution is to deposit a thin layer of a metal (or metals) prior to the deposition of the aluminum. This thin layer of metal, usually titanium, acts as a barrier or shunt layer. Therefore, if there is a small void in an aluminum interconnect, the current may go around the void by going down through the barrier layer instead. The result is a longer electromigration lifetime because it requires greater damage to the aluminum interconnect or line with the barrier layer in order to cause the same amount of damage to the electrical circuit without the barrier layer.
This solution, however, has several drawbacks. First, titanium has a much higher resistance than aluminum. Second, current barrier layers do not prevent hillocking at the sidewalls of an interconnect, so their effectiveness as a constraint is limited. A hillock is the localized displacement of a thin film material, such as aluminum, that occurs when the material is heated and cooled or when the material has undergone electromigration stressing. A hillock provides a location to accumulate the material that has been displaced when a void is created. Ease of hillocking is associated with degraded electromigration performance.
Another solution is to apply a mechanical constraint to the aluminum line or interconnect by passivation. This constraint is usually a layer of silicon dioxide deposited around the line which also electrically isolates one line from another. However, the industry is moving away from silicon dioxide and toward low temperature polymers as insulation/passivation layers in an effort to increase performance. This is because the use of the polymers as passivation layers has been ground to decrease the time delay of circuits. Unfortunately, these polymers are softer than silicon dioxide and thus, are unable to act as an effective mechanical constraint against electromigration induced hillocking.
Thus, what is needed is a process for cladding an interconnect to increase electromigration lifetime, prevent or reduce sidewall hillocking and allow the use of soft polymers to lower the RC (resistance-capacitance) time delay of a circuit.